TRANSITION-METAL (II) (III), EU(III), AND TB(III) IONS INDUCED MOLECULAR PHOTONIC OR GATES USING TRIANTHRYL CRYPTANDS OF VARYING CAVITY DIMENSION/

Three heteroditopic cryptands with different cavity dimensions have be en synthesized in high yields at 278 K without employing any templatin g metal ion. The three secondary amino nitrogens in each cryptand coul d be derivatized with anthryl groups to have a fluorophore-spacer-rece ptor configuration, The fluorophores in these systems do not show any fluorescence due to an efficient photoinduced intramolecular electron transfer (PET) from nitrogen lone pairs. However, the fluorescence can be recovered to different extents in the presence of different metal ions and protons as well. On complexation by a transition metal ion or on protonation in a solvent like dry THF, each exhibits large fluores cence enhancement as the nitrogen lone pairs responsible for PET are e ngaged in bonding. Inner-transition-metal ions like Eu(III) or Tb(III) show remarkable discrimination and give high fluorescence enhancement only in one case where the cavity size is smaller than that of other two. Each system exhibits large fluorescence enhancement with Pb(II) a mong the heavy metal ions studied. The present study shows that transi tion metal ions and Pb(II), which are known for quenching, can indeed cause fluorescence enhancement in cryptand-based systems. It is also r eported for the first time that inner-transition-metal ions can also c ause fluorescence. The enhancement in each case is interpreted in term s of a communication gap between the metal ion and fluorophore. Such c ryptand-based fluorophores can be useful as potential molecular photon ic devices and metal ion sensors as well.